In the fabrication of microelectronic interconnect substrates which typically use copper as the electrical conductor, a protective overcoat is often required because copper tends to be corroded by many hostile chemicals. This undesired corrosion degrades adhesion of the materials as well as other properties. (Polyimide, which is often used as the dielectric material, may not only contain moisture which can corrode the copper, but also attract copper diffusion.) In the fabrication of mulitlayer copper/dielectric interconnect substrates, the copper needs to be protected against corrosion and/or diffusion with an overcoat material such as nickel, chromium, titanium, or brown copper oxide.
In the past, difficulty has been experienced in accurately depositing electroless metal (or metal alloys) on closely defined copper features on a dielectric surface. There is a tendency for dielectric material to become sensitized and receive deposits of metal. Such undesired deposition of metal can cause short circuits between the circuitry elements. This constitutes a serious limitation of fabrication processes.
Prior methods which employed selective overplating of copper conductors with electroless metals were not very effective. One major problem that causes non-selective plating is the trapping of metal ions, such as palladium (II), from the activation process. The small amount of trapped metal ions on the dielectric surface can be reduced when contacted with an electroless plating solution. Depending on the circuitry complexity, it can be extremely difficult to remove the trapped ions by water or alkaline solutions. Potentially, the copper structure on the dielectric surface may block the hydrodynamic force during the rinsing process. In most cases, even a spin rinse is not effective; especially in the dense areas of the surface. Ultrasonic rinsing definitely assists the removal of these trapped ions, but also removes the palladium metal particles deposited on the copper surface. This then causes that section of the copper surface to become non-catalytic and results in uneven electroless plating. Alternate approaches have included the use of a plasma discharge to make the polyimide surface hydrophobic before soaking in an activator solution so that the metal ions are not trapped. The results, though, have been very unpredictable due to the uneven wettability of the surface. In addition, this pretreatment greatly degrades the adhesion between polyimide layers necessitating a further plasma discharge to remove the hydrophobic surface and ensure good adhesion between the polyimide layers. This increases the complexity of the fabrication process.
Prior attempts to solve these problems have included the following:
U.S. Pat. No. 4,810,332, which issued to J.D. Pan on Mar. 7, 1989 describes the use of thin polyimide layers and a metal electroplating mask for electroplating nickel on copper pillars. The major problems associated with this process are (i) the metal being undercut during the interconnect stripping process; (ii) shorting across the metal traces because the polyimide layer is too thin; (iii) shorting across the metal trace because of cracks in the polyimide; (iv) failure of the metal to plate due to organic scum; and (v) the copper is only partially protected due to incomplete coverage by the overcoat metal.
U.S. Pat. No. 4,770,899, which issued to F. M. Zeller on Sep. 13, 1988. This procedure provides for dipping of a palladium chloride catalyst solution and then subsequent dipping in a hydroxide solution with subsequent electroless plating of a metal. However, the hydroxide solution is not an efficient deactivator of polyimide and may only delay or reduce the tendency to plate polyimide for a short time.
U.S. Pat. No. 4,717,439 issued to J. Hajdu, et al, on Jan. 5, 1988, describes a process for preparing a copper oxide coating on a copper circuit board for leach resistance. This, however, is an overcoat solution only for processes that do not require high curing temperatures. This process might be useful in protecting copper in a plastic lamination process, but is not likely to protect copper if high temperature polyimide curing is subsequently required.
U.S. Pat. No. 4,568,562 which issued to E. Phillips on Feb. 4, 1986, describes a process which uses a tetrafluoromethane plasma treatment to passivate or neutralize the plastic substrate surface area in an attempt to selectively overcoat by electroless nickel. Unfortunately, since the copper is hydrophilic while the dielectric is hydrophobic, some activation solution may be trapped around the copper and difficult to rinse. Any such trapped activation solution can cause non-uniform plating. Also, subsequent application of additional plastic coats for multilevel fabrication may be complicated by poor adhesion to the modified and fluorinated surface.
U.S. Pat. No. 4,718,972 which to S. Babu on Jan. 12, 1988 uses an oxygen/halocarbon treatment to remove seed particles on the substrate. Similar drawbacks to those discussed for the U.S. Pat. No. 4,568,562 may arise. Furthermore, these two patents only solve part of the problem. After a plastic surface is made non-conducting, nickel plating on the plastic surface between copper lines (i.e. sheeting) can be caused by incomplete removal of catalytic species, such as palladium ions, which are introduced on to the plastic surface during the activation process.
U.S. Pat. No. 4,537,799 which issued to J. Dorey, et al, on Aug. 27, 1985 describes a process using acetic acid for treating a substrate having a negative mask to achieve selective metallization. Selectivity is achieved since acetic acid can only be effective to dissolved and/or modified negative photoresist type materials but not to any other materials such as thermosetting resins, thermoplastic resins and mixtures thereof.
U.S. Pat. No. 4,151,313 which issued to M. Wajima, et al, on Apr. 24, 1979, describes a method of modifying the negative working mask materials with solid solutions of oxides of metals to that the initiator for the electroless plating on the mask can be completely removed. This method is only applicable to the processes where modified materials are not permanent dielectrics and do not affect the electrical and mechanical properties of circuit boards after fabrication.
U.S. Pat. No. 3,640,765 which issued to R. DiStefano, et al, on Feb. 28, 1972, describes modifying sensitizing solution or activator solution with commercially available surfactants such as Triton X-102 or Triton N-57 to achieve plating selectivity between ceramic and resist. This, however, is not effective with metal/dielectric systems.
U.S. Pat. No. 3,443,988 which issued to J. McCormack, et al, on May 13, 1969, describes overlaying electroless nickel, cobalt, or polyalloys onto copper in an electronic circuitry pattern. Poison is added to modify the dielectric materials so that the catalytic activity can be decreased. A drawback to this procedure is that the poison compounds tend to be leached out after the dielectric is soaked in the plating bath for a certain period. A small trace amount of these poison compounds in the solution can terminate the electroless plating reaction. In addition, a trace amount of additives in the dielectric materials can dramatically change the dielectric's chemical, electrical, and mechanical properties.
U.S. Pat. No. 4,232,060 which issued to G. Mallory on Nov. 4, 1980, describes a procedure for formulating catalytic baths which form an adherent nickel or cobalt alloy film on the copper that was then overplated with electroless nickel. These baths are basically immersion baths using dimethylamine borane (DMAB) as a reducing agent. In principle, the activation energy for electroless nickel deposition with DMAB is lower than that for hypophosphite and thus some metals such as copper which are not catalytic in hypophosphite are possible to initiate plating reaction with DMAB baths. This approach, however, is not effective when the circuit density is high, and may also result in non-plating features.
None of these previous inventions solves the problem of selective electroless plating of an overcoat metal on a metal circuit with a minimum of time and expense. In addition, almost all fail to address the problem of current leakage on a plastic surface. The present invention provides a pre-treatment process which can be applied to a variety of technologies, including copper/dielectric material interface problems such as corrosion and interdiffusion.